There is a reason that we may need to store 'inactive' 200K accounts from legacy system that we need to replace.

It will all depend on the disk requirements, hence my questions around how many accounts can a CP-500-HW store. Is there any evidence from Aruba as to how much space is required for each arecord on the disk ?

It's just a database entry and it comes down to how long the username/password and any extra information that is stored with the users account. Every account is different size so there is no set account size calculation. The accounts will not be your issue it will be the logs and how much historical information you keep. Just remember you can only have 500 unique Mac address auth to a CP-500 in a 24 hour period